Chemistry of hydrocarbon compounds – Unsaturated compound synthesis – By dehydrogenation
Reexamination Certificate
2000-04-25
2002-12-24
Langel, Wayne A. (Department: 1754)
Chemistry of hydrocarbon compounds
Unsaturated compound synthesis
By dehydrogenation
C585S660000, C585S661000, C502S326000, C502S327000, C502S328000, C502S330000, C502S332000, C502S333000, C502S334000, C502S335000, C502S336000, C502S337000, C502S338000, C502S339000
Reexamination Certificate
active
06498280
ABSTRACT:
TECHNICAL FIELD
The invention relates to catalysts used in converting hydrocarbons and in particular to the paraffin dehydrogenation reaction. The invention pertains to a novel catalyst and to the preparation method carried out to synthesise it. The invention also relates to the use of said catalyst in paraffin dehydrogenation.
Alkenes constitute the feed of choice for the petrochemical industry. Steam cracking and catalytic cracking processes constitute the principle sources of alkenes. However, those two processes also produce by-products and the demand for specific alkenes which would be more expensive to produce by cracking is increasing.
For this reason, in some cases the direct production of alkenes remains an unavoidable step. This is the case with propylene, isobutene and long chain linear alkenes for producing polypropylene, MTBE and LAB (linear alkyl benzene) respectively.
The principal limitations of the dehydrogenation reaction are that the thermodynamic equilibrium limits the degree of conversion per pass and that the reaction is highly endothermic. These two features are determining factors in choosing process techniques and also in the design of the catalyst.
Thus n-paraffins containing 10 to 14 carbon atoms are generally dehydrogenated at temperatures of about 450-500° C. with a degree of conversion per pass in the range 10% to 25%, limited by the thermodynamics.
A high temperature operation is necessary to keep the degree of conversion close to the thermodynamic equilibrium, but a high temperature also encourages a certain number of side reactions, leading to a poor quality product. Such reactions include those leading to the formation of light products (cracking, hydrogenolysis), highly unsaturated compounds precursors of carbonaceous deposits, i.e., deactivation initiators (dehydrocyclisation, deep dehydrogenation) such as aromatic compounds or diolefins, and skeletal isomerisation reactions responsible for the formation of branched molecules. Because of such secondary reactions, under these particularly severe operating conditions it is very difficult to keep the activity high over long periods.
PRIOR ART
Means for limiting such secondary reactions can be aimed at the process and/or the catalytic formulation. Thus European patent EP-B1-0 462 094 claims adding hydrogen to the feed in H
2
/hydrocarbon mole ratios in the range 0.5 to 1.9. The purpose of adding hydrogen is to limit or retard the formation of coke on the catalyst surface without observing a negative effect on n-paraffin conversion.
A further solution proposed in U.S. Pat. Nos. 3,448,165, 3,907,921 and 5,233,118 consists of injecting a small quantity of water and/or sulphur with the hydrocarbon feed to be dehydrogenated. The water can be injected at a rate which is constant or which gradually increases with the catalyst function time. It was reported that an optimum as regards performance was obtained by increasing the injection of water with the temperature of the reactor during the operating cycle.
The other route which has been explored, again to improve the catalytic system performance, in particular stability, consists of determining the optimum physico-chemical properties. Thus U.S. Pat. No. 4,716,143 uses a catalyst based on a supported platinum such that the platinum distribution is limited to the external surface of the support over a maximum thickness of 400 &mgr;m. The advantage of such a choice resides in the fact that distribution at the periphery of the support can limit side reactions and as a result can improve catalyst performance. However, that type of distribution can only rarely produce platinum/modifier atomic ratios which are homogeneous on the particle scale (nanometers). Further, an excess concentration of active phase on the surface can cause diffusional limitations on the catalyst grain level (extragranular diffusion) and thus reduce the overall yield of the reaction.
The most frequently used platinum modifiers include elements from groups 14 and 13 and in particular tin (U.S. Pat. No. 3,745,112). The role of the tin present on the surface of the catalyst in the oxidation state +2, or more preferably +4, is to minimise the isomerisation and cracking reactions which occur at the acid sites of the support. A further example of a platinum modifier is indium, cited in particular in U.S. Pat. No. 4,551,574, European patent EP-B-0 183 861 and Japanese patent JP-B-91041211. Indium improves stability by also inhibiting secondary deep dehydrogenation reactions (polyolefins) and skeletal isomerisation reactions (branched hydrocarbons). It should be noted that the promoting power of such elements as regards platinum is also well known within the well-studied context of catalytic cracking catalysts.
As regards platinum based catalysts, because of the high cost of platinum, there is an interest in dispersing the metallic phase to the best extent, i.e., in increasing the proportion of noble metal in contact with the surface and the molecules to be transformed. It is important to develop a maximum specific metal surface area (surface area expressed per gram of metal) to obtain as high a degree of conversion as possible. Thus a drop in accessibility equivalent to an increase in particle size or a rearrangement of elementary particles is highly prejudicial to the productivity of the reaction. Thus one aims to minimise the particle size during preparation and to maintain this dispersion high, but thermodynamically unstable. The presence of a sufficient quantity of chlorine can constitute an answer to this problem. Chlorine is known to have a stabilising and even a re-dispersing effect on very small platinum particles. U.S. Pat. No. 4,430,517 cites an example of a chlorine content of more than 2.5% by weight for a platinum content of 0.75% by weight, corresponding to a Cl/Pt atomic ratio of about 19. That stabilising effect has also been known for a long time for catalytic reforming catalysts (U.S. Pat. Nos. 2,479,109, 2,602,772). However, while in the latter case, cyclisation and skeletal isomerisation reactions are highly desirable, in the case of dehydrogenation in general and dehydrogenation of paraffins containing 6 to 22 carbon atoms in particular, these reactions constitute side reactions which should be limited to avoid rapid deactivation of the catalyst. Thus adding chlorine causes a problem as a result of the secondary reactions which it encourages.
In order to limit secondary reactions, depositing an alkali or alkaline-earth, the role of which consists of contributing to neutralisation of the acid sites of the support with a weak and medium force, is important. Even a limited addition of lithium (0.1% by weight) can neutralise these acid sites which are responsible for the formation of isomerised and light products (cracking reactions). Aromatic compound formation is also reduced by adding lithium. However, this addition is also known to entrain a reduction in the total activity of the catalyst. This reduction is often linked to a phenomenon of coating the metallic phase with the alkali metal.
Conventional preparation methods cannot deposit sufficient amounts of alkali, in particular lithium, without producing a large drop in the accessibility of the platinum in the presence of a small quantity of a halogen. Studies in the literature have demonstrated this phenomenon when impregnating lithium into platinum based catalysts (Passos, Schmal, Frety, Catalysis Letters 14 (1994) 57-64).
To overcome this phenomenon, one route proposed in U.S. Pat. No. 5,536,695 consists of depositing lithium on an alumina support and carrying out a high temperature heat treatment to form a surface aluminate phase (LiAl
5
O
8
or LiAlO
2
). Platinum can then be deposited at the end of this step using a precursor, preferably an organic precursor. The use of an acid solution of a mineral precursor (hexachloroplatinic acid or hexahydroxyplatinic acid) suffers from the disadvantage of partially dissolving the aluminate support formed and thus leading to a loss of alkali metal. Further, inverting
Didillon Blaise
Pellier Emmanuel
Uzio Denis
Institut Francais du Pe'trole
Langel Wayne A.
Millen White Zelano & Branigan P.C.
Nguyen Cam N.
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